This invention relates to an improvement in extrusion heads, especially to those for use in coating applications. The invention is especially well adapted for application of adhesives in the manufacture of corrugated container board.
Paper coaters tend to be massive and expensive pieces of equipment. Both size and cost are a function of the requirements for rigidity and precision. A lower cost coater which is simple in concept has long been a need in the industry. Application of adhesive during the manufacture of corrugated board can be considered as a coating operation using a modified print-type coater. The adhesive is typically starch based and of relatively low viscosity.
A market need for corrugated board having improved moisture resistance has prompted a number of innovations using different board materials and adhesives. As examples, U.S. Pat. Nos. 3,518,142 and 3,972,763 show the use of hot melt adhesives for the manufacture of corrugated board. These adhesives are about two orders of magnitude higher viscosity than typical starch based adhesives and cannot be handled on conventional applicators designed for starch. In addition, they are much more expensive than starch. Economic considerations dictate that very low spreads are essential. Achieving good bond strength between corrugated medium and liners at very low adhesive spreads has been a major challenge not successfully met until the advent of the present invention.
United States patent applications, Ser. No. 836,181, filed Sept. 23, 1977 and Ser. No. 16,639 filed Mar. 1, 1979, now U.S. Pat. No. 4,223,633, commonly owned by the present assignee, show improvements in corrugator adhesive systems designed to better handle high viscosity hot melts. These applications are incorporated herein by reference.
One problem encountered with hot melt systems is oxidation and deterioration of the adhesive on exposure to air. Virtually, all adhesive applicators designed to apply hot melts in corrugating operations involve air exposure and recycling of much of the thus exposed adhesive. Oxidation results in the formation of gels that can plug filters or orifice nozzles on the glue applicator manifolds.
The desirability of direct application of a precisely metered amount of adhesive from an extruder onto the tips of the corrugated medium soon became apparent. This approach was suggested in U.S. Pat. No. 3,518,142. Unfortunately, there was no available system to perform this operation. A laboratory scale apparatus using a simple extrusion head showed that inordinately high amounts of adhesive were needed to get satisfactory bonds. Perusal of the literature showed a number of coaters with a raised downstream land or dam; e.g., U.S. Pat. Nos. 2,464,771 or 3,521,602, designed to promote coating uniformity. This approach on corrugator hot melt extrusion applicator gave improved results over a flush extruder face. However, useage of the expensive hot melt adhesives was still excessive if acceptable bond strengths were to be obtained. At least in part, this appeared to be a result of nonuniform adhesive distribution around the flute tip. The leading edges of the flutes appeared to be overspread while the trailing edges seemed starved of adhesive.